generics: can I require "comparable" together with some methods?

[Jochen Voss]

Hello,

I'm trying to write a generic function like the following (simplified) one:

type V interface {
    comparable // (1)
    SomeMethod(V)
}

func Test[val V](x val) {
    m := make(map[val]int) // (2)
    // uses a x as key in a map and calls x.SomeMethod()
}

When I try this, see https://go.dev/play/p/cJoH5YwEHaK , I get an error message for (1): "cannot use type V outside a type constraint: interface is (or embeds) comparable".  If I remove the line (1) I get an error for (2) instead: "invalid map key type val (missing comparable constraint)".

Is there a way to make something like this work?

Many thanks,

Jochen

[Ian Lance Taylor]

Are you using Go 1.20? See https://go.dev/blog/comparable .

Ian

[Jochen Voss]

Dear Ian,

Yes, I'm using 1.20.1.

I hadn't seen the blog post yet, will read this next.  Thanks for pointing me at this.  Judging from the first paragraph of the blog post alone, the change described there may not solve my problem, though.

All the best,

Jochen

[Jochen Voss]

Thanks again, reading the blog post indeed solved the problem!

On Friday, 17 February 2023 at 23:47:09 UTC Ian Lance Taylor wrote: